Various sensors such as pressure sensors, accelerometers, gyros, and microphones measure an external force by detecting the displacement of a mechanical structure. The displacement of this structure is converted to an external signal. However, variations in the sensor and the displacement detector can vary in time. For instance, accurate force measurement benefits from accurately knowing the spring constant k of the sensor. This constant can vary during operation due to, for example, variations in ambient conditions such as temperature, stress, thermal expansion and their effect on the spring constant. The spring constant can also vary due to permanent (irreversible) changes in the material and sensor properties, ageing, degradation, fatigue, contamination, threshold voltage shifts, charging and other mechanical and electronic variations in the sensor. Variations in manufacturing processes may also cause variations in the spring constant.
Due to variations that affect sensor operation including those discussed above, calibration can be important for ensuring sensor accuracy. Calibration often involves using a reference force of known value. However, it can be difficult to generate a constant reference force for a variety of applications. For example, electrostatic forces can drift significantly due to effects such as gap variation (e.g., due to temperature, stress or fatigue), charge accumulation and voltage drifts.
These and other matters have presented challenges to sensors and their implementation, for a variety of applications.